0
\$\begingroup\$

I am wanting to dim the light on my computers power light, its just a 5mm LED what i am guessing gets 5v from the motherboard to power it

The problem I am having is that when adding a 100R resistor in series with the LED it randomly turns on and off and sometimes flickers, then at other times it is fine

I tried with a different 68R resistor but the same issue occurs?

any ideas?

\$\endgroup\$
5
  • \$\begingroup\$ Sounds like a bad connection or a short. Or maybe it's actually the SSD/HDD activity LED. \$\endgroup\$ Jan 23 '20 at 7:26
  • \$\begingroup\$ I currently have a cable connected with no resistor, same 5mm LED and wiring and it works fine so i have it connected properly. I even tried a different carbon resistor and the same issue occurs \$\endgroup\$
    – alph
    Jan 23 '20 at 7:42
  • \$\begingroup\$ What voltages do you measure directly at the LED with and without the resistor? \$\endgroup\$ Jan 23 '20 at 7:47
  • \$\begingroup\$ Maybe try another LED. Sometimes they go bad internally. \$\endgroup\$ Jan 23 '20 at 7:50
  • 1
    \$\begingroup\$ If that is a red LED then you probably need a higher value resistor. While there are a vast number of "just a 5mm LED"s and none is strictly typical here is a typical one cree.com/led-components/media/documents/…. Vf = 2.1V typical and 2.6V max. At 2.1V across the LED at 5V I = V/R = (5-2.1)/100 = 29 mA. That's greater than the LED's rated 20 mA. The above LED can withstand 200 mA max BUT yours may not be able to. Try an eg 330 Ohm or similar resistor to start. \$\endgroup\$
    – Russell McMahon
    Jan 23 '20 at 10:40
2
\$\begingroup\$

If that is a red LED then you probably need a higher value resistor.

There are no "just a 5mm LED" typical LEDs.
While there are a vast number of 5mm LEDs, none is strictly typical.
Here is a 'typical' one that MAY be similar to yours, or quite different.
Vf = 2.1V typical and 2.6V max.
At 2.1V across the LED at 5V I = V/R = (5-2.1)/100 = 29 mA.
That's greater than the LED's typically rated rated 20 mA - and an 82R would result in higher current again.

The above LED can withstand 200 mA max BUT yours may not be able to.
Try an eg 330 Ohm or similar resistor to start.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.